1. Field of the Invention
The present invention relates to an optical data recording apparatus and an optical data recording/reproducing method performing servo control of a recording light beam.
2. Description of the Related Art
Generally, an optical recording medium has started to be used for recording a large amount of data since recording density of the optical recording medium is extremely higher than that of a magnetic recording medium. For example, an optical card has a storage capacity several thousand times to ten thousand times as compared with a magnetic card. Similar to a WORM type optical disk, the optical card is not rewritable, but the storage capacity is large, that is, 1 to 2M bytes. Therefore, it can be considered that the optical card is used in applications that preserve data on an individual heath management card, a prepaid card, and management of clients. The experiments in such applications have been already started in this technical field. Moreover, the use of the WORM type optical disk has been expanded in a document file, and the optical magnetic disk incorporated into a personal computer has been put on the market.
In an optical data recording/reproducing apparatus for an optical recording medium such as an optical card, an optical head is reciprocated relatively to the optical card to perform data recording/reproducing. Recording speed/reproducing speed is determined by a relative speed between the optical head and the optical card. Due to this, illuminating only a single track to record/reproduce data on/from the track cannot increase the recording speed/reproducing speed so much.
In order to solve the above problem, for example, as disclosed in U.S. Pat. No. 4,730,293, there is known a data recording/reproducing apparatus in which a plurality of tracks are simultaneously illuminated to read data from the plurality of tracks at the same time, thereby increasing the reading speed.
Moreover, for example, in "High Speed Optical Card Reader/Writer using Two Optical Sources" National Autumn Conf. Handout Document 1989 IECE Japan (Institute of Electronics and Communication Engineers of Japan), C-325, there is disclosed a data recording/reproducing apparatus using a different light source for recording and for reproducing. The structure of the optical head in the apparatus disclosed in the above document will be explained with references to FIGS. 1 to 3.
As shown in FIG. 1, a data recording section 4 and ID sections 6 are formed on an optical card 2 to be used in this apparatus. The data recording section 4 has a plurality of tracks extending in a longitudinal direction of the optical card 2 in parallel to each other. The ID sections 6 are arranged on both sides of the data recording section 4, respectively, and address data on each track is recorded therein.
On the other hand, as shown in FIG. 2, an optical head in this apparatus comprises a recording laser diode (hereinafter called recording LD) 10 serving as a light source for recording, and a reproducing LED 12 serving as a light source for reproducing.
Light emitted from the recording LD 10 is changed to a parallel light by a collimator lens 14. After the light is transmitted through a polarization beam splitter (PBS) 16, the light is focused by an objective lens 18 on the track of the optical card 2 serving as an optical recording medium.
Light emitted from the reproducing LED 12 is changed to a parallel light by a collimator lens 20. Thereafter, the parallel light is reflected by PBS 16, and focused on the track of the optical card 2 by the objective lens 18.
The light, which is reflected on the optical card 2 and sent from the reproducing LED 12, is changed to a parallel light by the objective lens 18, and partially reflected by PBS 16. Thereafter, the light is totally reflected by a mirror 22, transmitted through an image-forming lens 24, and totally reflected by a mirror 26 again. The light is split into light transmitted through a beam splitter (BS) 28, and light which is reflected on a beam split surface of BS 28, by the beam splitter BS 28. The transmitted light is inputted into a lead/tracking error detector (Tr-PD) 30, and the reflected light is inputted into a focus error detector (Fo-PD) 32.
The Tr-PD 30 comprises two triangle photodetectors 34 and 36 (hereinafter called "reproducing error device"), which are arranged such that their apexes are faced to each other and detect a tracking error at the time of reproduction, and two rectangular data detection devices 38 and 40, which are arranged at the upper and lower portions of the reproduction error devices 34 and 36.
A light spot irradiated on the optical card 2 is projected on the Tr-PD 30 as shown by reference numeral 42 in FIG. 3. An image 44a of a guide track 44 is formed on the reproduction error devices 34 and 36, and an image 46a of data pit 46 is formed on the data detecting device 40.
The detection of tracking error is performed by detecting the balance of light and shade of the image formed on the respective upper and lower reproduction error devices 34 and 36.
At this time, images of upper and lower tracks of the guide track 44 are formed on two data detection devices 38 and 40, thereby making it possible to obtain data of two tracks at the same time. It is noted that data recording is performed by the recording LD 10. In FIG. 3, a reference numeral 48 shows a light spot by the recording LD 10, that is, a recording light spot.
The detection of a focus error is performed by use of a reproduction light spot formed by the reproduction LED 12 shown in FIG. 2. In this case, the optical axis of the reproduction of light spot is shifted to the optical axis of the objective lens 18. The shift makes the light spot on the Fo-PD 32 move according to the movement of the objective lens 18. If the objective lens is moved to a lower portion in the figure, the spot formed on the Fo-PD 32 is moved right and left in the figure. The amount of movement of the spot on the Fo-FD 32 depends on the distance between the objective lens 18 and the optical card 2. This movement can be used for focus control. That is, the focus control can be performed by arranging the Fo-PD 32 at the position where the spot is formed in a proper focal position and by controlling the objective lens 18 such that the spot stays at the proper position.
However, even in the optical data recording/reproducing apparatus, the detection of a tracking error and that of a focus error are performed by use of the image formed by the reproduction LED 12. Due to this, it is important that there occurs no shift between a relative positional relationship between an optical spot 48 of the recording LD 10 and a light spot 49 of the reproduction LED 12 which is to be used for the tracking control and the focus control. The relative positional relationship between both spots 48 and 49 depends on the relative positional relationship between the recording LD 10 and the reproducing LED 12. The positional relationship between both light sources are influenced by a temperature change, a secular change, and others.
Even if the amount of the relative positional change between the recording LD and reproducing LED due to the temperature of the material where both light sources are provided is only 1 .mu.m to several .mu.m, the position, which is in a plane perpendicular to the track, of the optical spot formed by the recording LD is largely shifted.
For example, in the tracking, the light spot 48 of the recording LD 10 is not positioned at the center of the track and shifted up and down. If the light spot 48 is shifted, the recording position of the pit is also shifted. Generally, in the optical recording, the distance between the tracks is about several .mu.m to 10 .mu.m. Therefore, even if the above shift is not very large, the image of the pit overflows the reproducing devices 38 and 40 at the time of reproducing the recorded data and thus there is no possibility that data can be reproduced at worst.
In the focusing, the focal position of the light spot of the recording LD is largely shifted, the pit cannot be normally recorded and the recorded pit size is unstable. These points result in lowering reliability of recorded data and has a large influence on the recording/reproducing operation.
Moreover, there is a case that the optical axes of both light spots 48 and 49 to be focused on the optical card 2 are shifted due to assembly accuracy of the optical head. In this case, it is necessary to adjust the relative members in the optical head, resulting in increasing the cost and lowering reliability.
In order to solve the above disadvantages, it can be considered that the focus control and tracking control are performed by light emitted from the recording LD and reflected by the recording medium.
However, in this case, regarding the amount of the reflected light, the average amount of light to be emitted at the large output such that the light spot of the recording LD forms the pit is more than 10 times as much as the case where no recording is performed. Also, at the time of forming the pit on the recording medium, the change of reflection factor occurs by the pit and the reflected light is largely changed. Therefore, it is necessary to provide complicated circuits such as an AGC (auto gain control circuit) and a gain switching circuit in the focus control circuit and the tracking control circuit, and this causes an increase in manufacturing cost.